[HeyLaughingBoy]

I'll let you in on a secret: engineering the world over is mainly about rules of thumb and knowing when you need to actually do some math.

e.g., when I started my EE career, if you wanted to light up a red LED from 5V, you'd put a 330ohm resistor in series. If it was driven from 12V, then you'd use a 1kohm. Standard values that everyone has in inventory and you don't need to think about it. Similar "rules" would apply if you wanted to use an NPN transistor as a switch and so on.

Actual calculations would only come into play if I needed to e.g., make sure that the LED always had a constant 15mA through it whether the drive voltage was 5V or 24V.